1. Field of the Invention
The present invention relates to a method of coherent modulation and demodulation for HF (high frequency) data transmission at a high bit rate.
It can be applied notably to the digital tranmission of speech by radio.
2. Description of the Prior Art
It is known that, in ionospheric HF links, data is propagated by reflections on the layers of the ionosphere along multiple propagation paths. Owing to the turbulence of the ionospheric environment, the signal received for each of the paths varies randomly in amplitude and in phase. This prompts a phenomenon of fading of the composite signal received. Since the propagation times on each of the paths are dissimilar, the received signal is formed by several components spread out in time over an interval that may reach several milliseconds. Furthermore, the temporal variations of the heights of the ionospheric layers prompt frequency deviations that are characterized by Doppler shifts on each of the components of the multiple path.
All these effects are combined to produce a distortion of the signal and lower the quality of the link. The result of this is that transmissions of data at a high bit rate in the HF range are made particularly difficult. This is the reason for the introduction, historically, of parallel modulators-demodulators, also known as parallel modems, transmitting a large number of carriers in parallel, at low modulation speed. The most common type of modulation used is that of the M-ary differential phase-shift keying, which can be used to transmit several bits per symbol on each sub-carrier, in using a smaller bandwidth.
The signal emitted is then formed by a sequence of frames with a duration T equal to about 20 milliseconds, each frame being constituted by a sum of N sinusoidal waveforms at multiple frequencies of a quantity D.sub.f computed so as ensure the orthogonal quality of the sub-carriers for a duration of time T.sub.u smaller than the duration of the frame T.
The difference T.sub.g =T-T.sub.u defines a safety interval that makes it possible to prevent inter-symbol interference in the period of analysis T.sub.u. This makes it possible, in each frame, to separate different sub-carriers by a Fourier Transform and to demodulate them one by one. The modulation used on each sub-carrier is generally a binary or 4-ary differential phase shift keying.
One of the first HF transmission systems based on a parallel waveform modem is known from the article by R. R. Mosier and R. G. Clabaugh, "Kineplex, A Bandwidth Efficient Binary Transmission System" in the journal AIEE Trans, Part I, Communications and Electronics, 1958, 76, pp. 723-728. This modem, used for the point-to-point transmission of data, used 16 sub-channels at a bit rate of 75 bauds, with a 4-ary differential phase shift keying. The total bit rate achieved was 1400 bits per second. Another system known as "KATHRYN" has been developed by General Atronix in 1961 and is described in the article by P. R. Kirshal Gray and D. W. Hanna JR, "Field Test Result of the AN/GSC-10 Digital Data Terminal" in the journal IEEE Trans., 1968, COM-17, pp 118-128. This system enabled a modulation of 34 sub-carriers at 75 bauds. The modulation performed on each sub-carrier made it possible to measure the characteristic of the transmission channel for each of them and to correct the phase of each useful data element. The high performance characteristics of this method are however limited to slow fading and to a spread of the multiple paths that does not exceed one millisecond.
A new multitone method, known as codem, was subsequently developed in 1971 by General Atronix. This method, described in an article by D. Chase, "A Combinated Coding and Modulation Approach for Communications over Dispersive Channels" in the journal IEEE Transactions, 1973 COM-21, pp. 159-174, performed an M-ary differential phase-shift keying on a waveform formed by 25 orthogonal carriers using a weighted-decision error-correction code (25, 16) based on the amplitudes of the real and imaginary parts of the symbol, making it possible to reduce the effects of the selective "fading". Measurements made on this method have shown a gain in performance equal to about twice that of a 16-tone modem. The techniques used by the "codem" method were developed later in the context of the ANDVT (advanced neuroband digital voice terminal) standard, described in the article by W. M. Jewet and R. Cole JR. in NRL Memorandum Report 3811. They are applied in a modulator-demodulator optimized for digital phonic transmission with 39 tones spaced out at 56.25 Hz, with a useful frame duration equal to 17.8 ms; in 4-ary differential phase-shift keying (4-DPSK). In this modem, each frame formed by 39 symbols is transmitted at the bit rate of 44.44 Hz which corresponds to a duration of 22.5 ms divided into 17.8 ms of useful frame and 4.7 ms of safety interval. The total bit rate is about 1733.3 bauds and 3466.6 bits per second. At 2400 bits per second, the additional bit is used for protection with a redundancy of 2 of the 24 most significant bits of the phonic frame formed by 54 bits. In this coding, the safety interval of 4.7 ms and a sequential interleaving on the 34 tones made it possible to overcome the effects of ionospheric propagation.
Finally, in 1988, the Harris RF Communication group devised and perfected a multitone modem that had been already described by G. J. Luhowy and F. A. Perkins in "Advances in HF Technology", Harris Communication, 29 Sep. 1983. This modem is based on a 39-carrier parallel waveform. At 2400 bits per second, a Reed-Solomon code (14, 10, 2) and a temporal interleaving operation make it possible to minimize the influence of the multiple paths. For lower bit rates, stronger codes are used. In order to improve the precision of the phase reference for the phase demodulation, the HARRIS group has also developed a technique known as IPSK (Interpolated PSK) which can be used to obtain improvements as compared with the performance characteristics of a standard differential demodulation operation. In this method, the tones are alternately modulated with useful data and reference phases. In reception, the information on the phase reference is extracted from the reference tones, and an interpolation algorithm is used to obtain the values between these tones. The useful phases are determined by the difference between the interpolated reference phases and the values of the received phases.
While the advantage of the above-mentioned types of processing is that they can be implemented in a relatively simple way, they are nevertheless limited by a certain number of factors. First of all, the amplitude of the waveform emitted is not constant, and there is a ratio of about 10, in terms of decibels, between the peak power emitted and the mean power, although this result might have to be revised somewhat inasmuch as the modem generally undergoes a certain degree of clipping at emission. It also turns out to be the case that the modem is always highly sensitive to the selective fading in frequency produced by the multiple paths for the transfer function of the channel can always show deep fading at certain frequencies which lead to very high errors rates on the corresponding sub-carriers, although an error-correction coding and a frequency interleaving can be used to combat this phenomenon. Furthermore, differential demodulation always entails a loss of some decibels in comparison to coherent demodulation, this loss being about 2 decibels in non-coded QPSK on a white noise channel for example, although interpolation in the HARRIS modem makes it possible to reduce this loss. Finally, the lack of reliable information at the level of the demodulation prevents a weighted decoding of convolutional or other codes.